This invention relates to a system for removing snow and ice accumulation from concrete surfaces, and, more particularly, to a heated bridge deck system and the materials and method of fabrication and use.
Paved surfaces are prone to ice accumulation in winter weather. Concrete bridge decks are particularly vulnerable to icing in these conditions and also to frost formation in moderate temperatures since they are completely exposed to the air. Bridge decks generally freeze before the roads approaching them. Even slight ice accumulation on roadways can make driving treacherous. Statistics indicate that 10 to 15 percent of all roadway accidents are directly related to the roadway and its environment. This percentage alone represents thousands of human injuries and deaths and millions of dollars in property damage each year. Ice accumulation on paved surfaces is not merely a concern for motorists; icing of pedestrian walkways accounts for countless personal injuries, some potentially serious, due to slipping and falling.
In addition to natural melting and traffic movement, approaches to removing ice from paved roads and walkways traditionally involve mechanical treatments such as plowing. However, as the bond between ice and pavement can be quite strong, plowing alone may not be completely effective. In the alternative, road salts and chemicals for deicing are commonly applied to roadway ice accumulation. These chemicals melt into the ice and spread under the ice layer to help break the bond between the ice and the pavement. This can be rather effective, especially in conjunction with subsequent mechanical removal.
The most common deicing chemical used by highway agencies is sodium chloride (NaCl), commonly referred to as road salt. Road salt is also used to deice pedestrian walkways. It is usually used alone or mixed with fine granular particles such as sand. The temperature for effectively using road salt in deicing applications ranges from xe2x88x9210xc2x0 C. to 1xc2x0 C. (14xc2x0 F. to 34xc2x0 F.).
Another chemical frequently used in deicing operations is calcium chloride (CaCl2). Calcium chloride has qualities preferable to road salt in that it adheres better to paved surfaces at lower temperatures and has a freezing point below that of sodium chloride. One of the drawbacks of calcium chloride, however, is that it is more expensive than road salt. Therefore, rather than utilizing calcium chloride alone, it is often used in combination with road salt in low temperature (i.e., temperatures below xe2x88x9210xc2x0 C.) deicing operations. A further drawback is that the residual calcium chloride remains wet on the road surface, causing slick pavement. It also causes melted snow to re-freeze into ice when the temperature decreases.
The primary problems with using chloride salts as deicing agents involve the corrosive effects of the chloride ions present in the aforementioned chemicals. The use of chloride salts causes damage to concrete, corrosive damage to reinforcing steel in concrete bridge decks and other roadway structures, corrosive damage to automobile bodies, and pollution of roadside soils due to concentrations of sodium and chloride in water runoff. Furthermore, the use of salt produces osmotic pressure causing water to move toward the top layer of the pavement where freezing takes place. This action is more severe than ordinary seasonal freezing and thawing and causes greater stress to the surface of the pavement. These problems are a major concern to transportation officials and public works due to rapid degradation of existing concrete roadways and bridge decks.
Alternative chemicals which seek to replace chloride salts have been developed. Calcium magnesium acetate (CMA) is one alternative. Studies indicate that, unlike chloride salts, CMA is not likely to have an adverse effect on the environment. However, CMA is slower acting and less effective than chloride salts at lower temperatures, in freezing rain, in dry snow, and in light traffic. The application of CMA to the road surface also requires a larger truck capacity and larger enclosed storage space than chloride salts. Thus, CMA is a more expensive and less effective alternative to chloride salts.
Other deicing chemicals have been tested by various highway agencies with mixed results. Urea (CO(NH2)2, a soluble nitrogenous compound, is commonly used by airports as an ice control chemical due to it low corrosivity. However, urea is only effective at temperatures above xe2x88x929xc2x0 C. (15xc2x0 F.) and is less effective and more expensive than road salt. Magnesium chloride (MgCl2) is sometimes used as a substitute for calcium chloride because it is less expensive and works at similarly low temperatures. But, while it is effective in melting dry snow, magnesium chloride is less effective in melting ice. Formamide (NCONH2) is a less corrosive alternative to chloride salts but is much more expensive, and has a higher freezing point which lessens its effectiveness in colder temperatures. Finally, tetrapotassium pyrophosphate (TKPP) is an effective alternative for temperatures above xe2x88x924xc2x0 C. (25xc2x0 F.). TKPP has no corrosive effects on concrete and cannot penetrate concrete to affect reinforcing steel. However, it is corrosive to exposed steel (e.g., automobile chassis and brakes) and costs approximately 15 times as much as road salt.
In light of the drawbacks associated with road salts and chemicals for deicing, a significant amount of prior research has been directed toward developing a system for effectively preventing or removing roadway ice accumulation from paved surfaces without the detrimental effects associated with the use of chemical agents. This prior research primarily has centered on the use of both insulation materials for preventing ice accumulation and electric or thermal heating for deicing, but met only limited success.
Insulation of roadway structures and bridge decks is one method currently used to prevent frost and ice formation by reducing heat loss from the surface of the roadway or structure. As bridge decks are particularly prone to ice and frost formation, the underside of bridge decks have been insulated with materials such as urethane foam, plastic foam and polystyrene foam. A similar practice has been used in the subgrade of highway pavements and airfield runways. In addition to reducing heat loss from the surface and preventing ice and frost formation, insulation also seeks to decrease the number of seasonal freeze-thaw cycles to which the roadway or structure is subjected and also to decrease the amount of chemical deicing agent used to deice the roadway or structure.
Polystyrene foam insulation has been used in Michigan, Iowa, Minnesota and Alaska in the United States as well as Britain, Sweden and Canada. Results have shown that polystyrene effectively prevented frost formation in the subgrade of the roadways. Tests with urethane foam have been conducted in Missouri and Nebraska in the United States with mixed results. The urethane foam did help reduce the severity of frost and ice formation on roadways and bridge decks. However, it generally was not effective in achieving a reduction in the number of seasonal freeze-thaw cycles nor in reducing the amount of salt used in deicing applications thereon. Furthermore, there was a significant problem related to the bonding of urethane foam to the concrete.
Overall, insulation is only a partial solution to the deicing problem. Insulation is primarily used as a preventive measure, and can only prevent ice formation at certain temperatures. Once ice does accumulate on the roadway or structure, the insulation cannot be relied upon to remove the ice accumulation.
One viable solution to the remaining problem of removing ice accumulation involves the development of heating systems for roadways and structures. Obviously, by heating the surface of the roadway, structure, or bridge deck to a temperature above the freezing point of water (0xc2x0 C., 32xc2x0 F.), the snow and ice thereon will melt, alleviating the need for mechanical or chemical deicing agents.
Heating systems for use in pavements typically have been resistive electrical heaters or pipes containing heated fluid embedded in the pavement. The circulating fluid systems generally use fossil fuel energy sources. The use of low-grade, renewable thermal energy sources, such as geothermal water and the warm ground water below the frost line have also been tested.
The use of resistive electrical heaters embedded in a paved roadway has been tested in several states. In most applications, electrically heated cables are embedded throughout a layer of pavement. The natural resistance in the electrical cables heats the surrounding concrete and melts ice and snow atop the pavement. However, because the heating elements are embedded in the concrete, problems with them are nearly impossible to correct. Further, because the electrical heating elements have high power consumption, in certain instances the cost of electricity used to heat them is as high as $5.00/m2.
Pipes containing heated fluids also have been used to heat roadways. In most applications, a system of tubes is embedded throughout the concrete. Once the concrete has cured, heated fluids are circulated throughout the system of tubes, thus radiating heat throughout the paved surface. As with the resistive electrical elements, maintenance is nearly impossible in the event of a leak in a tube, and the costs of heating the fluid circulated throughout the tubes are quite high.
Experiments have also been conducted on the use of infrared lamps to heat bridge decks. It was discovered that this was not a viable alternative. One such system, employed in Denver, Colo., United States of America, used infrared heat lamps to heat the underside of a bridge deck. The system was found to be inadequate due to excessive lag time and insufficient power to provide effective deicing.
The use of gravity-operated heat pipes to transport thermal energy to a road surface also has been investigated. Systems of this nature depend on the condensation of an evaporated liquid and the latent heat of vaporization released during state change. Ammonia has been used as the working fluid in these heat pipes as it is not susceptible to freezing. These systems have been effective in sufficiently heating roadways and bridge decks to prevent freezing and to melt snow accumulation. However, these systems are quite complicated and expensive to construct and install. Roughly 40% of the total cost of these systems involves drilling and grouting evaporator pipes.
Thus, the use of insulation materials and electric or thermal heating has met limited success. Both techniques are not cost-effective to operate and are difficult to maintain satisfactory performance.
Each of the above methods for deicing roadways, bridge decks and other paved surfaces has its benefits as well as significant detriments. Therefore, a system is needed which will overcome the problems associated with the prior art methods and provide a uniformly heated paved surface, which is easy to install, and which can be operated in a cost-efficient manner.
Accordingly, it is an object of the present invention to reduce the accumulation of snow and ice on bridge decks, roadways and pedestrian thoroughfares.
It is another object of the invention to combine concrete and conductive materials so as to optimize the electrical conductivity of the concrete.
It is yet another object to make conductive concrete with uniformly distributed conductive materials.
Still another object of the present invention is to heat bridge decks, roadways and other paved surfaces in a manner which utilizes electrically conductive concrete coupled to a power source to generate heat.
A further object of the invention is to incorporate temperature and moisture sensors electrically coupled to a control unit for turning on and off the power source directed to the conductive concrete.
A still further object is to embed electrodes into conductive concrete to interact with the conductive materials therein and heat the concrete.
It is yet another object of the present invention to provide a system to create a heated paved surface comprising a first layer, a second layer made of an electrically conductive material situated atop said first layer, and means to apply electrical energy across said second layer.
Still another object of the invention is to provide a system for heating paved surfaces using a radio frequency or microwave energy source.
Another object is to disclose a system for heating paved surfaces using conductive concrete in the surface layer.
A further object to disclose a novel insulating material with a high level of mechanical strength and insulating capacity.
According to the present invention, the foregoing and other objects and advantages are attained by a conductive concrete mixture for use in a bridge deck system comprised of cement, aggregate, water and conductive materials. Preferably, the conductive materials are both metal fibers and metal particles and make up 1-3% and 5-40% respectively of the total volume of the mixture. More preferably, the metal fibers and metal particles make up 1-3% and 10-30% respectively of the total volume of the mixture. It is preferred that the mixture is used to manufacture pre-formed concrete slabs that have electrodes embedded therein, although a cast-in-place system is also a viable alternative and may be more cost-effective for existing bridge decks.
In accordance with a further aspect of the invention, a method of making conductive concrete comprises first mixing all fine materials (i.e., cement, Fly Ash, fine aggregates and Superplasticizer) with water in a container, subsequently loading coarse aggregate and metal particles onto a single conveyer from their respective containers, placing metal fibers onto the conveyer on top of the coarse aggregate and metal particles, emptying all of the contents of the conveyer into the container containing cement and water, and mixing the contents of the container.
In accordance with another aspect of the invention, a heating system for a bridge deck comprises a power source and conductive concrete forming at least a portion of the bridge deck electrically coupled to the power source.
A heating system for a bridge deck, in accordance with another aspect of the invention, comprises a photovoltaic cell, an energy storage device electrically coupled to the cell, and conductive concrete electrically coupled to the storage device forming at least a portion of the bridge deck. It is preferred that the energy storage device is a bank of one or more batteries. In an alternative aspect of the invention, the heating system may further comprise an inverter and a step-up transformer electrically coupled in sequence between the battery bank and the conductive concrete.
In accordance with a further aspect of the invention, a heating system for a bridge deck comprises conductive concrete forming at least a portion of the bridge deck, a power source electrically coupled to the concrete, a control unit for turning on and off the power source, and a temperature sensor and a moisture sensor electrically coupled to the control unit, wherein the power source is turned on or off upon sensing particular temperature and moisture levels. More than one temperature sensor may be provided, one for sensing air temperature and a second for sensing the surface temperature of the concrete.
In accordance with another aspect of the invention, electrodes for use in a conductive concrete bridge deck system comprise two parallel plate portions and at least one intermediate section between the parallel plate portions wherein at least one void is formed through which conductive concrete may flow. The parallel plate portions and the intermediate sections may be formed as part of a single metal plate. Alternatively, elongated rod structures may be attached to connect parallel plate portions formed of independent plates of smooth or corrugated metal.
In accordance with yet another aspect of the invention, a system to create a heated paved surface is comprised of a first layer, a second layer made of an electrically conductive material situated atop the first layer and a method for applying an electrical current to the second layer. A thermal insulating layer may be disposed between the first and second layers. The thermal insulating layer is preferably formed of 50-99% mortar by volume and 1-50% sawdust by volume. It is preferred that the second layer be comprised of a cementitious composite mixed with a plurality of electrically conductive components, for example, metal particles and fibers. It is further preferred that the method for applying electrical current to the second layer be sufficient to heat the surface to a temperature greater than 0xc2x0 C. An average electrical power of 500-600 W/m2 is generated.
In accordance with a further aspect of the invention, a system to melt ice and/or snow accumulation from a paved surface comprises a first layer, a second layer made of an electrically conductive material situated atop the first layer and a method for applying a radio frequency across the second layer sufficient to create microwave heating of the ice and/or snow accumulation on top of the second layer. A thermal insulating layer may be disposed between the first and second layers. The thermal insulating layer is preferably formed of 50-99% mortar by volume and 1-50% sawdust by volume.
In accordance with yet another aspect of the invention, a method to apply a paved surface capable of melting ice and/or snow accumulation from the surface thereof comprises applying a layer of electrically conductive material on top of an existing layer and applying an electrical current to the layer of electrically conductive material. A thermal insulation layer may be applied between the existing layer and the layer of electrically conductive material. It is preferred that the thermal insulating layer comprises 50-99% by volume mortar and 1-50% by volume sawdust. In accord with an alternative aspect of the invention, a radio frequency may be directed to the electrically conductive material rather than an electrical current. For either aspect, it is preferred that the electrically conductive material comprises a cementitious composite mixed with a plurality of electrically conductive components, e.g., metal particles and metal fibers.
Additional objects, advantages and novel features of the invention will be set forth in part in a description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.